In a recent medical field, endoscopes are widely used for diagnoses and treatment. In order to inspect an inside of a subject with use of the endoscope, white or broadband light is used for illumination in a normal light observation mode. Besides, narrowband light having a limited wavelength range comes to be more frequently used for the sake of emphasizing blood vessels in the subject in a special light observation mode, as disclosed in Japanese Patent No. 3559755.
As the wavelength of the light to be projected toward the inside of the subject becomes longer, a depth of reach of the light in the subject tissues increases. According to Japanese Patent No. 3559755, with use of the above characteristics, blood vessels at a particular depth are emphasized. For example, upon projection of blue (B) narrowband light having a shorter wavelength and a shorter depth of reach, the superficial blood vessels can be emphasized. Upon projection of green (G) narrowband light having a longer wavelength and a longer depth of reach than the B light, the middle-deep blood vessels can be emphasized more than the superficial blood vessels.
For endoscopic diagnosis, in accordance with a diagnostic situation, diagnosis is performed with focusing on middle-deep blood vessels in some cases, and diagnosis is performed with focusing on superficial blood vessels in other cases. Besides, diagnosis is performed with focusing on information on protrusions and recesses such as pit patterns in living tissues, in addition to the information on the blood vessels, in some other cases. Consequently, it has been demanded that only the middle-deep blood vessels or only the superficial blood vessels are emphasized without deleting the information on protrusions and recesses in the living tissues, on a case-by-case basis, namely, in accordance with the diagnostic purpose. Furthermore, in the case where both of the superficial blood vessels and the middle-deep blood vessels are conspicuous, it is demanded that, only the blood vessels to be focused on are emphasized, and the blood vessels not to be focused on are de-emphasized.
With regard to the above demand, according to the description of Japanese Patent No. 3559755, although it is possible to emphasize the blood vessels at a particular depth, there is no disclosure about de-emphasizing of the other blood vessels. Additionally, according to the description of Japanese Patent No. 3559755, since the illumination light has a limited wavelength range, visibility of the blood vessel itself is increased, but visibility of the information on protrusions and recesses in living tissues other than blood vessels may be decreased due to shortage of light amount.